privacy: add network connectivity checking toggle

NetworkManager supports toggling the periodic network check,
a check that by itself can be a security threat since it leaks
information about the host.

This patch adds a periodic network check toggle to the Privacy
panel.

https://bugzilla.gnome.org/show_bug.cgi?id=737362
11 jobs for connectivity in 6 minutes and 9 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Build
passed #189550
build

00:03:08

 
  Test
passed #189552
asan

00:03:00

passed #189551
test

00:02:37

 
  Manual
manual #189557
allowed to fail manual
armv8
manual #189553
allowed to fail manual
flatpak
manual #189556
allowed to fail manual
msan
manual #189558
allowed to fail manual
ppc64le
manual #189554
allowed to fail manual
tsan
manual #189555
allowed to fail manual
ubsan
 
  Review
manual #189559
allowed to fail manual
review
manual #189560
allowed to fail manual
stop_review